This idea goes back in 1967. James Lovelock, originator of the Gaia hypothesis, found a way to use a planetary atmosphere to detect life. He suggested that we look for simultaneous presence of pairs of gases like oxygen and methane that react together. We can also search for gases such as oxygen above levels expected from abiotic processes.

As far as we can see, Mars atmosphere seems to be close to equilibrium in this way. So when Viking I and II landed there in 1976, and found a barren desert-like surface, it seemed natural to conclude that there is no life on Mars.

However, just the following year, 1977, scientists trawling the ocean depths found life which could not be detected by his methods. Since then we've found many other ways life can get hidden from these observations of the atmosphere:

Deep underground

In caves

Past life that goes extinct

Life that flourishes every few million years like poppies in a desert

Sparse populations on the surface right now.

The last of these is the newest, sparking many researches into habitability of present day Mars over the last few years, since Phoenix. Life on a planetary surface could be hidden from these atmospheric measurements simply because of low populations, and a slow metabolism. For details, see "How much oxygen would present day life produce on the Mars surface" below.

But he didn't need any observations from close up. Its atmospheric composition is something we can measure from Earth, by clever disentangling of doppler shirts - a technique pioneered in the Lick Observatory observatory in 1926,

Lick observatory in 1900. First conclusive proof that there is almost no oxygen on Mars was a paper from this observatory in 1926

ATMOSPHERE OUT OF EQUILIBRIUM WITHOUT LIFE

James Lovelock's argument is not about oxygen alone, because it can be created abiotically.

For instance, on the early Mars, its low gravity makes it easy for the early oceans to evaporate - and then the hydrogen and oxygen dissociate - because it has almost no protection from solar radiation - and then the hydrogen could escape, leaving an oxygen rich atmosphere.

You wouldn't need much, not the 20% levels of Earth, to turn Mars red.

JAMES LOVELOCK'S LIFE DETECTION METHOD

His life detection method is based on search for methane and oxygen simultaneously, and other similar trace gases.

"The biological significance of an atmospheric mixture lies in the relative concentrations of a variety of constituents and not wholly in the presence or absence of any single one of them. Such a mixture is biologically significant if it represents a departure from a predictable abiological steady state. The strongest evidence is the simultaneous presence of two gases, like methane and oxygen, capable of undergoing . irreversible reaction; even with gases which can be expected to occur under abiological conditions, departures from the abundances to be anticipated will be of biological significance."

He also suggested that we search for life by looking at isotope ratios.

There is no doubt that his paper was an important contribution to the field - and it went on to become the foundation of his influential and important "Gaia" theory.

However almost immediately, scientists began to find ways that life could evade detection by James Lovelock's method.

FIRST WAY IT CAN BE INVISIBLE - LIFE WITH NO COMMUNICATION WITH THE SURFACE

Hydrothermal vents

First of all, just a few years later, scientists discovered the hydrothermal vent communities around the black and white "smokers" on the sea beds. They form at great depths, where the Earth's plates are pulled apart along the mid ocean ridges.

These were able to survive with almost no communication with the surface. The higher lifeforms there did use oxygen disolved in the water. Some of the microbes however didn't rely on anything except chemical energy and did not need oxygen.

Hydrothermal vent

Tubeworms deep under the ocean living on the hydrothermal vent Complex flow of chemicals for a hydrothermal vent. See Deep sea vents (Microbe wiki)

The interesting thing here is - that a planet could have communities like this at the depth of oceans - and they would have no effect at all on the atmosphere. Even the chemical biproducts just get disolved in the ocean and do not return to the atmosphere.

Cold seeps

This is just the first of many such discoveries.

Now we also have cold seeps, where microbes use methane and hydrogen sulfide as an energy source

Life in the rocks themselves

So - perhaps similarly Mars could have life - but only in caves or deep underground, in some way not connected to the surface.

MARS HYDROSPHERE

It might also have life in its hydrosphere. Unlike Earth, Mars has a cryosphere over its entire surface - a deep layer which remains at temperatures below freezing point of water. But deep down, temperature rizes, and a few kilometers below the surface - it gets warm enough - and also pressures high enough - so tht there may well be a layer of liquid water. Possibly a hundred meters thick, possibly deeper. This could be an ideal habitat for microbial life - almost completely disconnected from the surface.

Mars probably also has hot spots - as it has been geologically active in the recent geological past (though no confirmation yet of any present day hot spots and geological activity). If so - if the hot spot is beneath a "trapping layer" much like an oil field - it could provide another habitat with liquid water and almost no communication with the surface.

SNOWBALL OR SLUSHBALL EARTH

Then a bit later we discovered - that life doesn't need to cover a planet continuously.

Earth has gone through phases in the past when it was covered in ice, or almost completely so. This is the Snowball - or Slushball Earth

For millions of years - the entire surface of Earth -except possibly a small region of pack ice near the equator - was completely covered in ice. Was as barren as Antarctica - but without the penguins of course.

There was life there - but so little - that if you had landed a Viking probe on a random spot on Earth, chances are you would not have spotted a thing.

Once a planet is covered in ice - then it becomes highly reflective, it doesn't absorb as much heat from the sun - so it is pretty hard to get out of it.

Earth got out of this phase because of continental drift, then eventually limestone that deposited in the oceans during its warm phase gets subducted beneath the continents, and then returns to the atmosphere in volcanoes. With no life to turn that back into organics, and no oceans for the CO2 to disolve into - then the atmosphere got richer and richer in CO2

Finally the ice over the oceans began to melt, and life started to flourish again.

SNOWBALL MARS

This then raises the interesting idea - what if the same thing happened on Mars?

Scientists began to wonder if Mars might have had life in the past - now vanished. Or perhaps life there - but only occasionally depending on the tilt of its axis. Mars is dry right now, but might have shallow ponds occasionally as the atmosphere gets thicker.

At other times, it might survive as dormant spores, perhaps buried deep below the surface in caves or underground, protected from cosmic radiation.

We now have pretty conclusive evidence that Mars had a shallow ocean covering much of its northern hemisphere in the first few hundred million years of the solar system (and briefly again a billion years later).

At the very beginning, it's thought that Mars, Earth and Venus were almost identical with dense CO2 rich atmospheres of tens of atmospheres of pressure - which raised the boiling point of water permitting all three planets to have global oceans of water well above boiling point - planetary pressure cookers.

These atmospheres were also nitrogen rich - and the seas were rich in nitrates formed during asteroid impacts in the "Late heavy bombardment" - and probably also organics delivered by comets.

Mars is further out from the sun of course, and had no Moon impact. So life may have begun there earlier than on Earth. It cooled down more quickly however - and entered the snowball phase far sooner than Earth.

And - Mars has no continental drift. So unlike Earth which went through many snowball phases and survived - Mars just got colder and colder. There were temporary respites - floods perhaps due to giant impacts - also its axial tilt and orbit changes far more than the Earth's so that also led it's oceans to melt from time to time.

But before long it went into a permanent snowball phase. And then eventually - the ice disappeared as well - either lost to space - or underground - we don't know. At any rate it is currently as cold or colder than Snowball Earth ever was - but without any ice so dry as well.

WHAT WOULD HAPPEN TO LIFE ON PRESENT DAY MARS

So - let's suppose for purposes of discussion that Mars did have life in the early solar system? What would happen to it as a result of this snowball phase?

Well - to start with for sure, would be like Earth. There is life there - but it's hard to detect and mainly underground.

What little there was left on the surface - would have no detectable effect at all on the atmosphere.

If we were to spot an exoplanet like that around another star - then again - we would have no way of knowing that there is life there. And if you landed Curiosity on it - you would not expect to find anything.

Eventually - you might think - with the surface completely frozen, and no liquid water - it would be totally underground - so - this seems, on the face of it - to feed into the idea that Mars must be totally lifeless on the surface - and only have life deep underground.

However, various discoveries in Antarctica and in the permafrost layer have changed this picture

MARS CAN'T GET OUT OF ITS CURRENT "DRY SNOWBALL" PHASE THROUGH BIOLOGY.

There is no way that life on Mars could turn it back into a planet like Earth. It's too cold, too far from the sun, too dry etc.

Earth was only able to escape its snowball phase because of continental drift, which Mars doesn't have

But - it could have continued to survive, by "going slow" as the planet became less and less habitable.

Mars polar regions - one of the places where slowly metabolizing microbes may be able to survive, similar to P. cryohalolentis, an ordinary microbe but able to repair 10 DNA base pairs a year.

P. cryohalolentis can't survive 600,000 year's worth of damage on the Earth, butwas found still alive in 600,000 year old deposits in permafrost and Antarctic sea ice. The researchers believe it did this by metabolizing extremely slowly throughout the time period, using its ability to slowly repair DNA damage at about 10 base pairs of DNA a year.

This makes it a good candidate for a Mars microbe. The researchers said that it might also have reproduced occasionally - they couldn't tell - but it might have survived simply by using a slow metabolism + DNA repair. See Even Ordinary Microbes May Survive Radiation on Mars

LIFE THAT CAN SURVIVE IN TINY MICROHABITATS

Then we have had discoveries of possible habitats for life. I've covered this a fair bit in my other articles, and - one of the most promising are the warm seasonal flows, more of this in a minute.

But, let's just to mention the most recent discovery, by Nilton Renno's team.

They experimented in Mars simulation chambers found that they got tiny drops of water on the interface between salt and ice, in Mars like conditions. These could be widespread over the surface of Mars - but only as tiny droplets perhaps a few mm across, that form for a few hours a day, a few days a year.

""Based on the results of our experiment, we expect this soft ice that can liquify perhaps a few days per year, perhaps a few hours a day, almost anywhere on Mars. So going from mid lattitudes all the way to the polar regions. This is a small amount of liquid water. But for a bacteria, that would be a huge swimming pool - a little droplet of water is a huge amount of water for a bacteria. So, a small amount of water is enough for you to be able to create conditions for Mars to be habitable today'. And we believe this is possible in the shallow subsurface, and even the surface of the Mars polar region for a few hours per day during the spring." (transcript from 2 minutes into the video onwards)"

The researchers estimated oxygen production of the microbial mats at around 0.089 micrograms per square centimeter of surface per hour. That's 0.0089 grams per square kilometer per hour, or about 89 grams per square kilometer per year of oxygen production.

For the next step we need to know the "residence time" - how long the oxygen remains in the atmosphere. I can't find figures for Mars - anyone know (for methane it is 430 years)?

That is, of course, if the entire surface was as habitable as ice covered lakes in Antarctica.

How does that compare with the mass of the Mars atmosphere?

Using the average atmospheric pressure of 7 millibars, gravity 38% of Earth's, so 10 tons per square meter requires 26 tons per square meter on Mars, so 7 millibars requires about 0.182 tons per square meter, or about 182,000 tons per square kilometer.

So our 0.4 tons per square kilometer would contribute about 0.0002% by weight of oxygen to the Mars atmosphere.

As it turns out, Mars does have oxygen already, at far higher levels than that.

We don't really need to convert that to percentage by weight - is clear that Curiosity could not spot oxygen at 0.0002%. And diurnal cycles are going to be less than 0.4 parts per billion, at the limits of its sensitivity and surely hidden in the noise.

That is for the entire surface of Mars as productive as the Antarctic ice covered lakes - and all that oxygen ends up in the atmosphere.

HOW MUCH OXYGEN WOULD LIFE PRODUCE IN RARE HABITATS LIKE THE WARM SEASONAL FLOWS?

It's hardly likely that the surface has on average as much photosynthetic life as an ice covered lake in Antarctica.

Suppose that it only occurs in the warm seasonal flows?

These are exceedingly rare. Each asterisk on this map shows one of the sites

So there aren't many of them. And the habitats themselves consist of just thin streaks on the slopes like this

Suppose, for example, that we had a total of ten square kilometers surface area of these dark streaks on Mars - I mean just the streaks themselves. I expect that's an over estimate - but - this is just to give a very rough idea of what's involved here.

Suppose that they really are liquid water, as they seem to be - and suppose they are all inhabited by biofilm of life to a depth of a few mms as in Antarctica. And suppose these are all photosynthetic lifeforms.

Then that would be less than 1 kg of oxygen production over entire Mars surface, per year, and assuming a residence time of 4500 years, then a total of 4 tons of oxygen in the atmosphere.

At these levels, we have almost no chance of spotting Lovelock's traces of reactive gases.

Many other habitats suggested on Mars are also rare, such as the Martian Geysers. And others may be common - but consist of tiny droplets, like the ones found by Niton Tenno's team.

PROBLEM OF NITRATES

One major problem for life on Mars is the lack of nitrogen in the Mars atmosphere. Nitrogen is essential to life as we know it. However - Mars does have a bit of nitrogen in its atmosphere. And meteorite impacts continually deliver nitrates to Mars. Lightning can create nitrates also.

Another source of nitrogen - it might have deposits of nitrates from the early Mars. These might be deep underground, as happened in the Atacama desert, so not so obvious for our rovers to find.

This was just a hypothesis until last year -when Curiosity discovered the first conclusive evidence of nitrates on Mars.

So - it does have nitrates - patchy surely, low levels - but are present. And remember Curiosity can't drill even cms below the surface.

The McMurdo dry valleys and the Atacama deserts are similarly challenging in this way also - not much at all by way of nitrates - unless a micro-organism is nitrogen fixing.

Can any extremophiles handle nitrogen fixation in the Mars atmospheric condtions? I've asked around but nobody seems to have studied this.

An obvious follow up experiment would be to do the same tests, but with Antarctic nitrogen fixating extremophile micro-organisms. The authors of this paper propose the experiment to do that, but as far as I can see, haven't actually done it

However one way or another seems at least possible that surface life on Mars may have access to enough nitrogen, at least in places, at levels similar to nitrogen poor dry deserts on Earth..

WHAT ABOUT IONIZING RADIATION

Curiosity measured solar and cosmic radiation levels on the surface of Mars at roughly the same as the levels in the interior of the ISS. It is not particularly hostile for radioresistant microbes that wake up for a few hours a year. The most radioresistant Mars analogue organism, Chroococcidiopsis can repair tens of thousands of years worth of damage to its DNA in a few hours.

The Mars surface radiation is only hostile to dormant life. Over a billion years, then even the most radioresistant lifeform is reduced to almost nothing (1 in a trillion reduction in amino acids).

It does however mean that life needs to be there continuously. If there ever was a gap of time when the Mars surface became totally uninhabitable for a few million years, no life would survive on the surface, as far as we know, at least.

Of course that's the big question. If the habitats do exist, and there was life in them in the past - was it also able to survive continuouslyright up to the present day? Or were they reseeded by life from some other source at some point (underground, or from Earth say)? Or are they currently uninhabited?

ISOTOPE MEASUREMENTS OF LIQUID WATER ON MARS

Observations by the Phoenix lander show that present day Mars has liquid water on the surface - either all the time - or episodically in recent geological past. We know that because of isotope evidence of chemical exchange of oxygen atoms with the CO2 in the atmosphere.

That's far from actually proving that there is present day life there - or that there are habitats there - but it leaves the possibility open at least

DOES MARS HAVE PRESENT DAY, OR PAST LIFE?

Perhaps Phoenix detected evidence of the Warm Seasonal Flows - and Nilton Renno's "swimming pools for bacteria. Or perhaps it detected evidence of occasional impacts or volcanic hot spots that liberate floods of water onto the surface?

If these habitats do exist - is there any life in them? Or - has life on Mars gone extinct?

Or does it still exist but only underground, or on the surface, but only occasionally?

Nobody knows at present.

When searching for sparse communities contributing at most a few tons of oxygen to the atmosphere, in total, over the entire surface of Mars then we may have to go right up close to the microbes to spot anything. Even then, they might be hard to spot.

Then another complication, with such sparse habitats and slowly metabolizing life - how long does it take life to "discover" a new habitat on Mars? On Earth it happens within weeks and months, habitats created in volcanic eruptions are colonized almost immediately.

On Mars, this process could easily take centuries, millennia, or even longer. So it's quite possible that some of the habitats are inhabited, and other, equally good habitats, have no life in them.

COMMENTS

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Comments

Here's a SETI talk, from yesterday, describing how those dark streaks on Mars likely were caused by frozen CO2 rather than by water. It is just a detail in your reasoning, but since you mention those streaks and this is spot on that subject, and I saw this a moment before I read your blog, here it is:http://www.seti.org/weeky-lecture/investigations-strange-linear-features...

Here is a movie of the warm seasonal flows - notice how they are much flatter - no 3D structure - just a darkening of the slope basically, also they change seasonally, this is a single season. Get gradually longer in the spring. In autumn they fade away.

Why this is, nobody knows. It's not likely to be damp sand directly, because there wouldn't be enough water to make it as dark as this. So some secondary effect of some sort.

But - because of the temperature, and because there is no correlation at all with wind, or dust, and impossible to be dry ice because of the warmth, and the seasonal nature of it - they have run out of hypotheses. The only one that makes much sense is water - though that one is also quite hard to fit to the phenomenon.

The problem with water is that it needs to be replenished. Even tiny amounts of water like this - if it's been going on for millions of years, then the resevoir of ice or water would get exhausted, at the top of the slope, and is no rain - no mists here either (are at lower elevation), no way for the water to get back up to the top of the slope. So how did it get there in the first place?

The ice could have got there, perhaps tens or hundreds of millions of years ago during one of the periodic changes of the Mars climate because of changing axial tilt and changing orbital eccentricity etc. But that's so long ago - is surprising there is any water left.

It could be deliquescing salts - perhaps it comes, originally, from the high humidity in the night time atmosphere on Mars. After all you don't need that much water to feed the streaks, probably.

Or maybe it comes from deep below the surface? Maybe these are places where the land is somewhat warmer, a geological hot spot? That's one possible theory, so that the ice rises, sublimates, freezes, sublimates, and freezes, perhaps all the way to the surface from the deeper hydrosphere, over a geological hot spot.

Whatever the explanation, another puzzling thing is - that you get apparently identical spots, apparently, and some have these streaks and some don't, and nobody knows why that is.

It could be perhaps because of subsurface salt deposits - at the head of the streaks - (just a short way below the surface) - or geological hot spots - or some other unusual local phenomenon we can't see from orbit, which somehow feeds new water (in tiny quantities) to the heads of the streaks each spring.

They look quite similar to warm seasonal flows, don't they? But they are much broader, and to experts in the field, only superficially similar.

They may be caused by avalanches of dry sand, perhaps in combination with some ice / water effects. I think you can see they don't look quite the same - and they don't have the same patterns of seasonal variation as the warm seasonal flows.

So we have

Linear Gullies (CO2 blocks)

Dark Streaks (Dry sand avalanches)

Warm Seasonal flows (Water of some kind only half way convincing hypothesis so far)

And - quite a few other ways that the surface of Mars changes right now. By far the majority of the surface is completely unchanged from year to year, except for the dust storms and movements of sand. But we do get:

Martian Geysers (CO2)

The related "Dark dune spots"

Dry gullies (CO2 phenomenon)

Moving sand dunes and other sand patterns

Fresh craters

Those are all things that can change at least from one year to the next.Some of them might have life - the dark dune spots particularly - they are striking to look at, resemble patterns of life. That doesn't really mean anything of course - but it is one possible hypothesis for them, that along with the inorganic processes that some liquid water might get involved along with the dry ice, and provide a habitat for life.Moving sand dunes could also provide a habitat for life by "churning up" the chemicals in the soil, bringing salts and ice to the surface to create temporary habitats for life, the "advancing sand dunes bioreactor" theory.

You're certainly well informed on many many Martian things! Your above summary of these kinds of flow formations is great for a member of the casually interested audience, like me. I've stumbled upon Cosmic Diary, a blog mostly about sand dunes on Mars, another kind of phenomena with baffling variation, but likely of less importance to the life issue. At least I've never seen any such enlightening overview about the potential significance of dunes to life.

It might though just possibly have life involved as well, by the solid state greenhouse effect.

Like this, first the dark spot forms - this is thought to be an entirely dry ice phenomenon - this is in the colder Southern polar cap which has thick layers of dry ice in winter. So - the idea is the sun shines through the translucent dry ice, heating up lower layers which then turn directly into gas. Pressure builds up until eventually the sub surface dry ice pocket "explodes" to the surface as a geyser.

But as it gets warmer, in the second, third and fourth images, then water brines may form.

This causes "Flow Like Features" or FLF for short, which you can see here, gradual progression from left to right in the sequence of images. This is in the South polar region.

These features grow at a rate of around 1.4 meters per Martian sol.

They model these using a solid state greenhouse effect, but this time one that involves translucent ice rather than dry ice as the solid state greenhouse.

The way it works is, according to their model - that the ice forms a thin translucent layer - and as it warms up in summer - then the ice melts a layer of water beneath it, a process that is reasonably well known on the Earth.

In their model the ice, even in the polar regions, reaches 0°C, enough to melt ice to water. Then as it flows down the slope, and cools down, it mixes with salts to form salty brines.

On Mars this process is especially interesting, as it is a way for the water to stay liquid under pressure. After all, the dry ice geysers have CO2 gas sublimate under enough pressure to escape from the surface as geysers.

I haven't read the original papers here, yet, just Nilton Renno's survey - and he doesn't go into a huge amount of detail - but I think that must be why (at the head of the flow) water could be liquid in these habitats at temperatures that would cause it to evaporate quickly in any other surface habitat.

So, the source of the water is particularly warm at0°C - making the flows quite a promising habitat for life on Mars.

Then - a slightly different process seems to form flow like features in the Northern polar region, this time using the solid state greenhouse effect in dry ice to create the brines themselves:

The flows increase at between 0.3 metersand 7 metersa day. The feature marked with the arrows grows at a rate of about 5 meters a day.

Here, the features form at around -90°C for the surface temperatures. You might think that is too cold for liquid water, even as brines. But in their models they did get liquid water beneath a layer of dry ice - as the thermal infra red penetrates the dry ice. The orbiting satellites are not able to see this increase in temperature because their resolution is too low at this wavelength to pick out such small features amongst the much colder surrounding dry ice landscape.

One of the research papers on these features postulates dry ice and sand cascading down the slope but most of the models involve liquid brines.

TRANSGRESSING SAND DUNES

Then - there's this paper from last year, HABITABILITY OF TRANGRESSING MARS DUNES which suggests that the sand dunes churn up salt layers and bring them to the surface - where they might then provide habitats for life by deliquescence.

That one is unusual as it is one of the few suggestions for ways you could have life on Mars even in the dry equatorial regions where Curiosity is right now.

Gilbert Levin of course remains sure that Viking discovered life. And, he is no longer a "lone voice" saying this, a few others have started to wonder if just possibly they did.

If that is life - then that means life at Vikings location - which is surprising as it is very dry - but does have its morning frosts. And does have night time humidity 100% obviously.

There are the DLR experiments also, with lichens and cyanobacteria able to continue to metabolize almost anywhere on Mars - though not done very long experiments yet last time I read about it, months rather than years of Mars surface conditions simulation. And is a major challenge to simulate everything we know about surface of Mars in a laboratory. But - if they are right - then there could be life anywhere including equatorial regions and so in the sand-dunes - again depending on availabiity of nutrients.

If otherwise habitable, sand dunes seem interesting as a way to supply nutrients to life, because it gives an opportunity for materials to be churned to the surface from deeper deposits.

Then, Nilton Temmo's "swimming pools for bacteria" - they could be anywhere on Mars so including in the sand dunes, so long as it is far enough from equator to have ice, and so long as there are salt deposits as well.

So, it is certainly still not ruled out that there is life on Mars today. And it might be dangerous to bring it to Earth. I probably should not put this idea in your mind, if you haven't had it yourself already, but maybe Mars (and most of the universe) is almost barren to life precisely because it is infected with an extremely dangerous anti-life bug!!!

However, bringing soil samples from Mars to Earth has the great benefit of our huge biological laboratories. Whatever we can send to Mars or even to a quarantine in LEO, at any time in the next centuries, will be very much inferior to small robotic bio labs on Earth at that time.

Subjectively, you seem to be more careful while I am more curious. Carefulness hinders curiosity, while curiosity might cause carelessness. Let's try to find some rational approach to deal with this issue.

A) The easy scenario to deal with, I think, is the case where the small (just one or a few tons each) bio labs we can send to Mars in our lifetime, do detect the Earth like life they are able to look for! In that case I think that:

1- Further robotic analysis can be targeted according to what we've learned about Martian life and will likely continue to be very successful even without needing the huge bio labs on Earth. Carefulness and curiosity can go hand in hand.

2- Sample return from Mars to Earth should be strictly forbidden since alien life which is like us is likely enough to harm us that we should not allow it.

3- Humans, or any other Earth life beyond very safe and well motivated experiments, should not be sent to Mars because they might contaminate the alien life and destroy the information we want to study.

B) The difficult scenario is if a dozen small robotic bio labs detect no trace of current life during the next several decades. As you keep pointing out, there might even then be life on Mars. Either in unexamined pockets, or life of such exotic kinds that our bio detectors fail to react to it. Then what to do? I think:

1- We should give a go ahead for sending humans to Mars, for whatever purpose people want to go there with their pets and grow their carrots in greenhouses or whatever. If Martian life is either so hidden or so exotic that our robots haven't found it by then, it is unlikely to be globally contaminated by a small human colony. They might rather contribute to discovering hidden life if any.

2- If after a few years of a Mars "colony" no effects on humans or plants have been noticed, samples and humans should be allowed to return from Mars to Earth for proper analysis in our best labs.

C) But, maybe there is a quick and safe test of the risks of sample return!?

Then dock the Mars samples returned with those Earth Simulating Capsules. Btw, we could call them ESCapes, as in landscapes (with and without apes...) and expose them maximally to each other. If no harm is done to any of those ESCapes, we can conclude that it is safe to bring Mars soil samples to Earth and our huge and ever improving bio labs here where we might find fossils or harmless active life. We could still do it cautiously, but we should do it.

Okay, yes that's a curious idea, not come across it before, the idea that most of the universe is infected with a bug that makes e.g. higher life like ours really hard to develop :). Is that why it took so long for multi-cellular life to develop here...

Anyway, yes we are both agreed, returning samples to Earth has huge benefits when we can do it safely with our big biological laboratories.

For example - well maybe it's possible I hesitate to say anything can't be done - but not seen any suggestions of a way we can do accelerator mass spectrometry on a small robot in space

It's not that big, after all, they've managed to miniaturize electron microscopes....

But anyway - that's a digression, there will always be bigger and more powerful instruments back here on Earth - and also - means you can send bits of the sample to many different labs for independent verification of results and stuff like that.

A) The easy scenario to deal with, I think, is the case where the small (just one or a few tons each) bio labs we can send to Mars in our lifetime, do detect the Earth like life they are able to look for! In that case I think that:

1- Further robotic analysis can be targeted according to what we've learned about Martian life and will likely continue to be very successful even without needing the huge bio labs on Earth. Carefulness and curiosity can go hand in hand.2- Sample return from Mars to Earth should be strictly forbidden since alien life which is like us is likely enough to harm us that we should not allow it.3- Humans, or any other Earth life beyond very safe and well motivated experiments, should not be sent to Mars because they might contaminate the alien life and destroy the information we want to study.

Okay - I do hope others will think the same way you do here. For the reasons of the likes of XNA or whatever it is could be returned from Mars. Perhaps most people would start to think this way if we detect life on Mars. By far the safest for sure.

B) The difficult scenario is if a dozen small robotic bio labs detect no trace of current life during the next several decades. As you keep pointing out, there might even then be life on Mars. Either in unexamined pockets, or life of such exotic kinds that our bio detectors fail to react to it. Then what to do? I think:

1- We should give a go ahead for sending humans to Mars, for whatever purpose people want to go there with their pets and grow their carrots in greenhouses or whatever. If Martian life is either so hidden or so exotic that our robots haven't found it by then, it is unlikely to be globally contaminated by a small human colony. They might rather contribute to discovering hidden life if any.2- If after a few years of a Mars "colony" no effects on humans or plants have been noticed, samples and humans should be allowed to return from Mars to Earth for proper analysis in our best labs.

I have a few problems with this. Indeed I think this is the most difficult scenario all round. Because we don't know what is there at all. Haven't yet got a clue except that if there is life there, it is rare.

IF LIFE IS RARE DOESN'T MAKE IT SAFE FOR HUMANS

You have all the problems you had before of XNA etc - except - you never know when it might strike.

RARE MARS LIFE LIKELY TO BE MORE VULNERABLE TO EARTH LIFE IF ANYTHING

It might be that for whatever reason you have just a few species that have somehow survived. E.g. ur ancestor type species not as adapted as modern life, but somewhere on Mars surface they have managed to hang on somehow. Or - just that they are pretty well adapted but not many because conditions there are so hostile.

Just because it is rare doesn't follow that Earth life can't overwhelm it.

For an easy example - suppose that Mars life hasn't yet developed photosynthesis. We have no idea how easy or hard it is to develop photosynthesis evolutionarily. Just chemosynthesis.

Well it could be very rare. Perfectly adapted to Mars. But terribly vulnerable to introduced photosynthetic life that would spread over Mars and take over from it. Likes of Chroococcidiopsis.

In the other direction Mars life could have developed some other capability, like photosynthesis, that no Earth life has yet developed - even - a more efficient way of doing photosynthesis. That could mean it takes over from photosynthetic life on Earth if introduced here.

DOZEN ROBOTIC LABS NOT NEARLY ENOUGH

A dozen small robotic bio labs is a major plus, but nowhere near enough. We have at least half a dozen different possible habitats for present day life after all already.In no particular order:

Warm seasonal flows

Transgressing sand dunes

Dark dune spots

Pockets of water on ice / salt interfaces

Solid state greenhouse effect beneath ice sheets

Life on the surface using humidity of the atmosphere

And most of the missions in near future will be searching for past life - the present day life more likely in the higher lattitudes.

Then also - places like the warm seasonal flows - they are on steep slopes - likely to be hard to get to.

And - given how rare the warm seasonal flows are - and quite probably not all identical in habitability - and also that the habitable ones may not all be inhabited - I think ideally we should send rovers to each one (that is of course we don't find life on our first attempt - if we do - then still would send rovers to each one eventually but that's your first scenario).

I see more like - I don't know, maybe 40 or 50 robotic missions, to have a half decent chance. But that's just a start really. I think can't say in advance. Just explore until the exobiologists say they have a reasonably thorough understanding of Mars from a biological point of view. Which may take some time.

GLOBAL CONTAMINATION

And - I can't agree that it is unlikely to be globally contaminated. The problem being, introduce something like, say, Chroococcidiopsis and it may be able to survive almost anywhere on Mars.

And - once introduced, however slow the spread, as with Carl Sagan's single microbe producing soil like Earth over entire Mars surface if not limited by conditions (which of course it would be) after ten years of doubling once a month.

The big thing is - the dust storms. A human base would be shedding huge numbers of microbes, some in dormant state, and these would get into the dust and get spread in the dust storms over large areas of Mars. Just needs a warm seasonal flow somewhere in the track of a dust storm that goes over a human base first, and you risk contaminating it with Earth life, possibly quite early on.

HAZARDS FOR HUMANS

Also - if there is XNA on Mars - well it is no less hazardous to humans because it is rare and hasn't been discovered yet. So problem there is that eventually some of the humans would return to Earth as technology evolves. And could bring it back with them.

And - XNA life could be - so small it is not recognized as life. And it could have an incubation period.

INCUBATION PERIOD

This is the really tough thing, with back contamination. That some infectious diseases for instance have an incubation period of decades. Could be the same with the returned life - I know remote chance it infects humans, but if it does - they could be just fine for decades.

And on return to Earth also -safe for humans on Mars doesn't mean safe for Earth.

And problems in ecosystem again - could build up slowly over decades. E.g. spreads in the soil, some life form that out competes Earth life, but only in - say higher lattitudes and only a slight edge over the Earth metabolism. So - gradually spreads but is a very slow exponential. So first year almost no effect, first five years still a small colony you hardly notice - but then ten years later BOOM.

Or changes in response to conditions that occur rarely. E.g. locusts, you would never guess that a locust is harmful if you haven't seen them in a swarm. But then something or other triggers them to change behaviour, and BOOM again.

It's really tricky this incubation period thing.

Personally I'm not sure that incubation really tells you much at all. Not unless we know what the life is like already.

I think all of this needs to come after we discover the life, not before. And based on careful study of it.

Of course, as in our other discussions, I'm not saying that any of this is probable or likely - but that as responsible humans we have to take account of them - and if it's potentially damaging to the Earth even if 1 in a million chance, or less, we need to involve all those potentially affected by it, can't make the decision for them.

IRREVERSIBLE EFFECT ON MARS

But finally also - it is just that introducing life to a planet is irreversible. There is absolutely no way that we can predict what effect it would have - on a planet so different as Mars. Is not like introducing potatoes to the old world for instance.

Or - it might be - but it might not be. We just don't know enough.

So - not saying we never do this, just the idea of doing a multi step forward plannign right now, based on the idea that it must be okay to introduce life to Mars if it is currently devoid of life as far as we know - I don't think we should do that.

C) But, maybe there is a quick and safe test of the risks of sample return!?

Let's send to a cheap but safe orbit (such as DRO, Lunar Distant Retrograde Orbit), not huge labs, but modest capsules which contain fresh samples which reconstruct several different biological environment from Earth. Atmospheric, oceanic, underground, arctic, jungle whatever types of biotopes and microbial biomes and even simulate Earth's non-biological minerals, gasses, gravity, radiation.Then dock the Mars samples returned with those Earth Simulating Capsules. Btw, we could call them ESCapes, as in landscapes (with and without apes...) and expose them maximally to each other. If no harm is done to any of those ESCapes, we can conclude that it is safe to bring Mars soil samples to Earth and our huge and ever improving bio labs here where we might find fossils or harmless active life. We could still do it cautiously, but we should do it.

The problem here is, the incubation period, and special conditions. Plus impossibility of testing all possible habitats on Earth.

E.g. - what if it is harmful to crops, or animals, or what if it interferes with the ecosystem in a more subtle way...

It's a nice idea, sure. But I'd need more than that to be at all confident that it is okay.

I think safest is just to continue to explore "in situ" on Mars, and hope we find life early, because that then will make it much easier to decide what to do next, but if not, keep searching for some time.

And if it was up to me I'd put a complete moratorium on humans on the surface, indefinitely - until our understanding changes in some way so we have some reasonable idea what we are doing.

But whatever - that has to be a decision we make as a planet, and individual companies and countries shouldn't do that, I think just about anyone who writes on this agrees that.

That is unless it is possible for humans to explore Mars in a clean and biologically reversible way - but I can't see how that is possible now - we used to think it was possible about 6 years ago. Would need to do a lot of work to convince the experts that it is possible now I'd imagine if you had a COSPAR review of it, including taking account of hard landing etc

Anyway - sorry I haven't written that updated back contamination article yet. Maybe that will be my next article (though I have a couple of others I want to do also but it would be a good one to do).

1) Small probabilities drown in large ones and are therefor rational to ignore. What if we all die in an ancient Earthly plague virus dug up (much more likely than getting exterminated by some returned Martian soil bug), while spending centuries avoiding efficient study of potential Martian biology, which actually might've learned us how to avoid such an extinction!?

2) Microbes with long incubation times, haven't they evolved intimately together with us since ages? And wouldn't decades of incubation be easily detected in labs on Earth? A catastrophe evolving from a Mars return sample would have to happen very quickly and surprisingly to be really devastating.

3) Only human individuals have minds. Nothing else can think. The "collective planet" has no idea, there is no such thing as a collective mind. The parole "we collectively" has murdered 100 000 000 innocent civilians during the last century, not even counting any soldiers. But single individuals thinking and acting on their own have at the same time brought science and ideas and tools which has saved the lives of billions of us.

I am afraid that my impression remains that you are guided by an irrational emotional fear of investigating exobiology. You propose sterilizing samples of exolife in orbit. I propose instead to fertilize it in orbit to test your hypothesis of it being hostile. But you still refer to probabilities in the shadow of many other more important dangers. I urge you to try to find a solution to your own conundrum of wanting to learn without wanting to touch.

Well the thing is - is an introduced microbe from Mars introducing something new into the situation unlike the hazards we get on Earth, that's the big question.

I'd say it does for many reasons, which I've already given -

XNA possiblity,

GTAs,

Pathogens

Creates biochemicals that closely resemble Earth life biochemicals but behave slightly differently and so poison us or our animals or plants (I think a real possibility for an alien biochemistry).

Allergens - trigger immune system - like peanut butter - not toxic but kills some people

may have traits such as more efficient metabolism, better photosynthesis etc so that they out compete Earth life

Of course could be the same in the other direction and a mixture also, some Mars life better at some things and in some habitats than Earth life, and vice versa.

I've said that already so don't want to repeat it, in detail, just a short reminder, will do a proper article about it, have also mentioned it in the other articles here.

So I think the main thing here - to go into more details about the quarantine idea problem. Because many people have suggested this. But when you look at it closer, it's really hard to see how it can be got to work.

NATURAL CONTAMINATION STANDARD IDEA

One of the authors on this subject suggested a possible approach, the "natural contamination standard" which I think is what you are talking about.

His idea is - that if we can reduce the problem so that it is no more hazardous than the meteorites we receive from Mars anyway - then it is okay - even though that is well above, probably, the 1 in a trillion trillion type probabilities you'd get the other way by thinking through the chance it might be XNA and existential risk type calculations.

The problem there of course is, how do you achieve the natural contamination standard. It is fine in its way but no good if there is no way to achieve it.

My idea of sterilizing it with ionizing radiation - which you don't seem to like for some reason - might be one way to do that.

Certainly, just returning it directly from the surface within a year or two in a protected container - and doing the best you can to preserve Mars atmosphere etc probably too - is far too different from meteorite impacts with their impacts every million years or so, most into basalt and such like, most not likely to transfer surface dust and salt, and with most of the material taking millions of years in transit etc etc.

PROBLEMS WITH QUARANTINE FOR PATHOGENS

Now - the problem with quarantine - the thing is - is okay if you know a fair bit about what it is and why it might be dangerous. E.g. we can quarantine dogs against rabies because we know the latency and incubation periods for rabies.

But when it is unknown - we know there is a chance of a Martian microbe infecting humans, because of the analogy of Legionnaires disease - a disease of amoeba that uses essentially the same mechanism to infect humans. So similarly a disease of Mars microbes - just possibly - might be a pathogen for humans - or be able to adapt to become a pathogen for us - or for our animals, - with small adapations in near future.

That has a short incubation period of 2-14 days. But leprosy has a longer one of decades. And Legionnaires disease did not evolve together with us for a long time, is a recent disease of humans. So - expect at least some latency period, question is how long, and I'm no expert but Carl Sagan looked into it - is not much known but though mostly known as a popularizer he was also a pioneering researcher in the emerging field of exobiology with background in both astronomy and biology. So well qualified in this area.

So this is Carl Sagan's argument, that by analogy with Leprosy, that we can't assign a fixed quarantine period of a few years and say that if nothing happens to a human within, say, 10 years of quarantine, that it is safe (never mind the few weeks of Apollo).

As for the Apollo regulations - way out of date - and not scrutinized by biologists outside of those who set them - were published on the very day that Apollo 11 was launched (that would not be permitted today). So - can't go back and say that because they used quarantine, that quarantine makes sense as a method of planetary protection - the Apollo quarantine idea essentially had no peer review.

At the very least - you need a modern exobiologist to look into it thoroughly and see if he or she can come up with any reason for setting an upper bound on the latency period of a pathogen from Mars. Given that it will have some sort of latency or incubation period, question is just, how long it is and can you give it an upper bound.

ETHICAL PROBLEMS OF HUMAN QUARANTINE AGAINST UNKNOWN PATHOGENS OR POISONS

For that matter your mini ecosystems can't contain humans.

The problem there is - what do you do if your human gets ill? If he or she is a volunteer and dies of a Martian pathogen, or allergen, or some poison produced as a biproduct - that's one thing. But what if they just get ill?

And - if they die - not instantly - would be a long drawn out illness chances are. Are you, seriously, gong to let them just die slowly, not sure if it is a Mars effect, or maybe some ordinary Earth illness?

In practise would be an outcry if that happened, huge lobby to return your astronaut for treatment, which is what would happen - and - that then means that essentially your quarantine has added no extra safety level at all.

Human quarantine just doesn't really work ethically even if you had a limited quarantine period. Well that's my view at least. It just can't be done ethically.

PROBLEM FOR QUARANTINE FOR DETECTING ISSUES IN ECOSYSTEM

Well again you have latency period. In animals and other creatures. Your microbes might have latency and incubancy periods in animals also. And in the soil.

And when populations grow, you get a slow exponential growth to start with.

It's okay if it is like the red or blue curves. But what if it is like the green curve, and the numbers here are decades?

Then - I should have flagged this as my own idea, you also have the likes of locusts

Whee the relevant point here is - that it happens almost without warning, some small trigger that sets them into this swarming behaviour.

If you had a few locusts in a quarantine habitat in orbit around the Moon, you simply would never discover this.

Now not saying that there are locusts on Mars.

But may well be microbes that are fine to start with. But then they develop a new adaptation, or some trigger in the environment, and they move rapidly to a problem.

Remember also that these may be archaea, swapping genes. So the danger might not be a Mars microbe as such, but some hybrid of a Mars microbe with a totally unrelated Earth archaea.

And the sudden "locust swarm" event could be the result of a gene getting transferred from a Mars microbe to an Earth one or vice versa.

With your 3) - actually by "the world" there I meant humans. That the decision has to involve all humans - or at least people who have the authority to act as their legitimate representatives in the debate - and in this age of internet and easy communication - should be encouraged to talk over these issues and debate it widely. As we are doing indeed.

And final decision shouldn't be made by just one company or country. That's all I'm saying and just about everyone who has written anything on back contamination agrees here. Except those who think it is a total non issue like Zubrin of course.

So - I think myself, quarantine is useless, really. It just might turn up a problem if there is one. But is no good at all for showing that there isn't a problem.

That's not at all obvious. Plenty of good people have suggested quarantine. I only realized these issues myself after reading what Carl Sagan said about it and other articles which I can't remember now, and then thinking through the implications and realized that indeed, it just doesn't work for the reasons just given.

So - either just continue doing robotic exploration. Or if you do return a sample, then sterilize it.

And the problem is - returning a sample before you study it.

I say, that frustrating though it may be, you study it first and return it - thoroughly study it - and as you say if you find life in it - then will be obvious to most people that we need to take great care.

Or sterilize it thoroughly in some way that preserves biosignatures and geological interst of the sample but makes it impossible for life to survive the sterilization.

Or - can return to L1 or L2 if you are totally sure not going to hit the Earth - and if you make totally sure also that humans don't handle it directly until thoroughly understood and comes nowhere near the Earth or human inhabited habitats.

Or - can return to Earth if TOTALLY SURE - that you can't have any escape of Mars particles. But there I'm not at all sure you can achieve even a 1 in a million probability of escape of a 10 nanometer particle from the capsule into the Earth environment when it is opened (given that optical resolution is 200 nm for diffraction limited microscopes) and think also the various accidents and human error and other risks of breach of containment would raise it to quite a bit above a 1 in a million chance, especially with many missions to Mars failing totally, and many missions also have flaws in the design not discovered until later.

I see those as the only real possibilities. At present. Until we know more about what we are doing here.

You have proven, to yourself, that humanity must never ever find out whether there is life on Mars or not. You have meticulously excluded every possibility of doing so. Unfortunately, your great interest and knowledge, and many great ideas, about Mars are overcome by some irrational fearful emotion which has gotten the better of you. Only God can be TOTALLY SURE, and He does not exist, so...

If you imagine that there is such extremely potent life on Mars, could you formulate any kind of probability measure of it by remotely observing, say, a billion exoplanets without any trace of civilization or atmospheric life? Let's hear you explain that Mars likely is dangerously alive, while the entire universe is dead.

And let us see how you prove that there cannot be alien dangerous life evolving under our feet right now here inside Earth. What should we do to protect us against that? We surely can't escape to mars...

I've noticed that you have no recognition of the concept "economy", so this will certainly be a totally incomprehensible category of concept to you, but here we go:
Have you ever weighted the pro's against the con's? What benefits could we get from learning about an independent biosphere? Is it at all possible for you to imagine that it could be worth taking the risk???

(Btw, look out, are you TOTALLY SURE that you will not be hit by a meteor any second now!!!)

That's not what I think at all. And is also a view shared by many people so if you think it is irrational, you have to say that, for instance, Carl Sagan was also irrational in the same way. Because he voiced exactly the same concerns.

I'm saying that we need to study "in situ" first as the safest way to do it - and also - as the way that makes most sense biologically.

All these arguments are to do with people who want to return life to Earth before they study it.

E.g. the idea that you just return a sample of Mars dust or soil to a habitat in orbit and quarantine that habitat and by seeing what happens establish if it is safe at all. So, I'm saying that does not work.

Not saying that you don't study Mars at all! Not saying you never return a sample!

Just saying that a particular idea, the quarantine idea, doesn't work and pointing out some particular issues in that idea.

And I've suggested several different ways we can do it.

Study in Situ all the way through as in your 1.

I think this is by far the best way of discovering past and present day life.

For all the reasons that you can choose what to sample based on your previous observations, and can do many samples, far more than you can return to Earth do the equivalent of thousands of sample returns in a single mission.

Because unless your rover is equipped to detect life, it has no way of knowing if the samples contain life anyway in the first place.

It is just too expensive and inefficient anyway - to return geologically interesting samples that probably have no life in them, even if they have organics, organics don't mean life yet - as those exobiologists argued in their white paper for the decadal review.

Sterilize enough so it can have no reproducing life before studying

I think this is a poor second best to in situ search for life. But given that NASA seem to have sample return as their priority, may be worth thinking about whether that can be done safely all the same.

- I'm not saying here - sterilize all samples from Mars in perpetuity - that might be part of the mis communication? I'm saying sterilize the first samples until we have a chance to study it

It's just return of unknown, not yet studied (in biological sense) samples to Earth that is the really big issue here.

Return to L1 or L2

Return to Earth receiving facility - and rely on containment.

It is just in this case where, I personally would need to be very sure that it is safe. Carl Sagan again said the same thing here - that perhaps it is safe to return to a biohazard faciility - but he would need to be very sure that it was safe.

And I gave several reasons why - because of the risks of human error, errors in design of the mission, accidents, natural disasters, and the very hard problem of making sure that everythingis sealed to 10 nm scale when diffraction limited optical microscopes can only see down to 200 nm.

So - that's all I'm saying.And presenting my personal view in spirit of debate. Which we should have world wide if it ever becomes something in the near future. Will be many views and since it is our lives that are being risked, not just theirs then we need to be part of the decision.

This is what the ESF study said

RECOMMENDATION 10: Considering the global nature of the issue, consequences resulting from an unintended release could be borne by a larger set of countries than those involved in the programme. It is recommended that mechanisms dedicated to ethical and social issues of the risks and benefits raised by an MSR are set up at the international level and are open to representatives of all countries.

"the problem of risk - even extremely low risk - is exacerbated because the consequences of back contamination could be quite severe Without being overly dramatic, the consequences might well include the extinction of species and the destruction of whole ecosystems. Humans could also be threatened with death or a significant decrease in life prospects

In this situation, what is an ethically acceptable level of risk, even if it is quite low? This is not a technical question for scientists and engineers. Rather, it is a moral question concerning risk. Currently, the vast majority of the people exposed to this risk do not have a voice or a vote in the decision to accept it. Most of the literature, on back contamination is framed as a discourse among experts in planetary protection. Yet, as I've already argued, space exploration is inescapably a social endeavor done on behalf of the human race. Astronauts and all the supporting engineers and scientists work as representatives of the human race...

..In this situation, to treat persons with dignity and justice means that everyone must have an opportunity to voice their opinion concerning whether humans should accept the risk..."

I'm just saying the same as this. And as for risks versus benefits - well scientists tend to weigh the benefits of an experiment higher than the informed public, so - they are not the best ones to make a decision on this.

As he says

This is not a technical question for scientists and engineers. Rather, it is a moral question concerning risk.

That takes you into ethics. And scientists have no special position in their ethical decisions. They do when it comes to finding out the data you need to make the decisions. But when it comes to the decision itself, they don't. Not when it implicates the whole population of the Earth.

So, if you think it is worth taking the risk to return a sample to Earth, because of the science return you expect from it - well that's your view, that's fine you are entitled to have whatever view you have on it! Many scientists will share this view.Including, saying that we shouldn't take any precautions, which I think is Zubrin's view on this.It's only if you or anyone else go on were to say that you are entitled to make the decision for everyone else because you believe your view is the only correct one. Especially if anyone says they have special status in this decision because they understand the science better. They don't, not in an ethical decision.

That's when it goes a bit too far.

So - to make it clear - and this is just my view for debate, not meant to say anyone else has to think this way:

My own personal view is that an uncontained or poorly contained sample return BEFORE IT HAS BEEN STUDIED FOR SIGNS OF LIFE - fails on two counts

Poor return on the money spent for its science value because there is a good chance it doesn't have life in it, just organics

Not ethically sound because even a low probability of life, say 10% combined with difficulty of containing it results in an unacceptable probability of risk even though it is small

While in situ sample study, just in my own view, out performs it both ways, better science return, for less cost, and no ethical issues at all as nothing is returned to Earth.

And I think quarantine, though it seems like a great idea at first - if you work through the details - does not work as a way of containing an unknown hazard in a not yet studied sample.

If you still think the quarantine approach will work, am interested to hear if you have any answers to the issues I raised.

Robert, you are absolutely correct. Despite contrary claims, there is no acceptable risk when operating in ignorance. The simple truth is that we aren't even confident how Earth microbes will respond to a prolonged space flight, since their behavior/traits in space is different from that on the surface.

As a result, one can't be sure that any organism found on the Martian surface would be returned to Earth with the same traits as it had there. The notion that life on Earth has some intrinsic advantage because it evolved here is simply another variation of the "naturalistic fallacy", or "an appeal to nature". If a lifeform is compatible with the life on Earth, then there is no way to know that it wouldn't become an invasive species, being suddenly introduced into more favorable conditions.

There are no rational decisions in the face of total ignorance, and our ignorance of Martian life [should it exist] is total at this point.

Thanks, exactly, you've put it far better than I did. It's an argument from ignorance, that because we don't know if there is Mars life or what it is that we don't need to bother about it.There's an important clause in the Precautionary principle, as described in the Wingspread conference.,[6] a key principle in political decision making, and law:

When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.

In this context the proponent of an activity, rather than the public, should bear the burden of proof.

The process of applying the Precautionary Principle must be open, informed and democratic and must include potentially affected parties. It must also involve an examination of the full range of alternatives, including no action [6]

There the important point is "even if some cause and effect relationships are not fully established scientifically".

I.e. one should not deduce safety of a procedure from ignorance of its outcome.

Yes for sure, not that it is likely it can be changed at this late date.

Quoting from you think you put it so well :)

If a lifeform is compatible with the life on Earth, then there is no way to know that it wouldn't become an invasive species, being suddenly introduced into more favorable conditions.There are no rational decisions in the face of total ignorance, and our ignorance of Martian life [should it exist] is total at this point.

Another important point, that it says that

It must also involve an examination of the full range of alternatives, including no action

"recommend the level of assurance for the exclusion of an unintended release of a potential Mars life form into the Earth's biosphere for a Mars Sample Return mission".

For them "no action" as in, not returning a sample to the Earth at all, wasn't an option within their remit.

Indeed, don't think I've seen a big study with that as a remit, to examine different alternatives and see which has best science return and which is safest and to balance those issues together.

That's been done at another level, at the NASA planning stage, informed by the decadal review.And - never been any major study considering the alternative of an in situ study and comparing it with sample return.

ONLY PUBLISHED STUDY I KNOW COMPARING IN SITU ASTROBIOLOGY ON MARS WITH SAMPLE RETURN

"Currently, MSR is regarded by much of the scientific community as largely weighted towards a technology demonstration as the rationale for good astrobiology will not be apparent until we discover more about our neighboring planet."

...

"We argue here that when in situ methods have definitively identified biomarkers, or when all reasonable in situ technologies have been exhausted, it would be time for MSR. We are not yet at that crossroad."

Of course that's before Curiosity's discovery of organics - but surprise rather is we didn't find it earlier and organics are just the first step, Mars never have had life, or it degraded over billions of years enough so that it is unrecognizable through various processes - and as for present day life - then expected to need to search many samples even in an inhabited region before you detect it in desert conditions like Mars - so don't think that changes much (something apparently is destroying the influx of meteorite organics we expected to find on the surface).

So, as it is I think that in situ is better scientifically anyway, at least I haven't seen anything published yet to suggest otherwise.

A big list of advantages of returning a sample to Earth (plenty of papers and studies that start like this) is of no interest if it doesn't also investigate what are the chances of that sample containing biologically interesting material,

Needs to be a proper head to head comparison with in situ. Comparing costs, combined with science return - with in situ studies and taking account of the many new instruments we can send to Mars now to investigate it biologically in situ.

And we don't have to do a "no action" alternative - as in not search for life at all, as "in situ" has no ethical conflicts here. So I don't personally have any conflict here between science and ethics. At least pending future more extensive comparisons of the two.

But if there was a conflict, ethics wins every time in my book. Even though I love science!

I've just added a more detailed description now to that comment, explaining it properly and going into the details of the Flow Like Features that form around the Dark Dune Spots - and why the main models for them so far involve liquid water. It is just summarizing the Dark Dune Spots + FLF section of Nilton Renno's paper in less technical language, but might be useful all the same. Direct link to the comment

Oh, did you think I meant that as a reason for it being life... Not at all of course many non life like processes look like life. It was just echoing what many people say as their first impression - and famously of course Arthur C. Clarke referred to similar patterns as "banyans on Mars" - he actually thought it must be life.

However, the reason those dark dune spots are included in the lists of present day possible candidates for habitats on Mars for present day life is not at all because of the life like appearance. It's because - although mainly a dry ice pheneomenon, there seems to be some possibility of liquid water as well (small quantities obviously) due to the solid state greenhouse effect there. Nilton Tenno covers the dark dune spots in his overview paper of the possible present day habitats for life on Mars.

Standing Space, I had a go at a draft of the sample return / in situ article but was a bit dry and encyclopedia like, so didn't publish it.

Am going to have another go though.

First, though, I think I should do an article on the latest on potential habitats for life on Mars surface and shallow sub surface (top 2 or 3 cms above the permafrost layer).

It would also go into the various Mars analogue microbes on Earth - and the experiments in exposing them to Mars analogue environments on the ISS and on the ground.

All that helps to motivate it, is because of the potential for life on the surface of Mars or subsurface that could be returned in a MSR that you need caution in the first place. If you think there is no present day life on the surface of Mars, and probably none in the subsurface except near rare geological hot spots (as nearly everyone thought about six years ago), obviously going to be less concerned about a Mars sample return.

Also is a good article to do anyway, is a while since I wrote about that, in a rapidly developing field, and a fun article to write - and - I don't know of a good overview article on this topic anywhere in non technical language to link to.

Nilton Renno's one is a good overview of course from 2013 but is too technical for most readers.

Wikipedia is poor on this subject - as I said, with an editorial viewpoint in many of the articles that life can't exist in these habitats.

And Encyclopedia Britannica has only a brief mention, all it says is "Alternatively, it could be argued that the best strategy is to look for present-day life in niches, such as warm volcanic regions or the intermittent flows of what may be briny water, in the hope that life, if it ever started on Mars, would survive where conditions were hospitable."

where that "intermittent flows of what may be briny water" covers the entire field of present day habitability of the Mars surface and top few cms of sub surface.

And are plenty of news stories about individual potential habitats, e.g. the stories about Nilton Remmo's announcement, or the ones about the DLR findings, or articles about the Warm Seasonal Flows - but none of them attempt an overview of them all.

If anyone knows of any other non technical overview of this field, do say! I've searched, and don't know why it hasn't been done.

So will do that first. Then as a separate article, for the back contamination, going to focus instead on which is most effective for exobiology, in situ first or sample return first. Again that's a question that just doesn't seem to be asked (similarly to the question, are telerobots or robots controlled from Earth or humans best for exploring Mars, nobody does the comparison except in the Herro study which came strongly in favour of telerobots in comparison with humans - it's just like that in this field, many fundamental questions that don't seem to get asked much, and big policy decisions made without asking basic questions about why you are doing it in the first place).

Only comparison study I know is that white paper for the decadal review, and as they didn't mention it in the summing up, I can only conclude that, despite giving exobiology as the motivation, exobiology can't really be their top priority for the mission (otherwise the white paper would be top of their discussion agenda).

I think, as they said in that paper, it is more of a technology demo. Also it may be a response to the Safe on Mars report in 2002 which recommended a sample return from Mars to show safety for human missions to Mars. But if that is any part of the motivation, it is way way out of date in a rapidly evolving field - and I don't see how a sample return nowadays from a few locations selected just because e.g. they contain organics, could show safety for humans on Mars or safety of Mars from humans. So if that is their motivation, then after a sample return, one that probably has no life in it, don't see how they can conclude anything. Especially if ExoMars in its continuing program has detected life in similar locations on Mars by then.

Anyway - will chase that up a bit. Then my ionizing radiation suggestion comes in after that - if it is indeed a technology demo not expected to contain life - and if you accept that it is not likely to be of any value for showing that Mars is safe for astronauts or astronauts safe for Mars - then why not just sterilize the entire sample for safety reasons?

And suggest - that if the aim on the other hand is to find out whether there is life on Mars, then given that there are at least two ways of doing it, and the only comparison study to date has shown in situ to be far better - you should do a new comparison study and use that as a basis for the decision.

Plus the various other things from our conversation here, all set out properly in order.

Plus the material on need for public to make the final ethical decisions rather than the scientists, in cases like this - where everyone is affected, and if general public has different views on it from scientists. E.g. if an experiment has a small chance that it could lead to a "discovery of the century" - does that justify e.g. a less than 1 in a million (or 1 in a billion or even 1 in a trillion or whatever) chance of invasive species or environmental disruption of the Earth? Is that enough assurance for it to go ahead? Many scientists, especially those hoping to make the discovery, might sometimes say Yes in a situation like this when general public might say no. In that case then general public should decide, not the scientists, many thinkers at least say.

And will make clear where I'm presented generally agreed ideas, where it is controversial and there are several viewpoints - and where I'm presenting an opinion of my own for discussion e.g. the sterilizing the whole sample suggestion is obviously my own suggestion for discussion.